Gravure printing and coating methods are well known means of applying liquids to webs or sheets. U.S. Pat. No. 4,373,443 describes the use of a gravure cylinder to provide ink in newspaper presses. Engraved upon the surface of the gravure cylinder are cells or depressions that are filled to excess with coating liquid. Commonly, a gravure cylinder 10 rotates in a pan 21 holding a constant level of coating liquid that exits via a drainage port 26 for wetting, as shown in FIG. 1 and as taught for example in U.S. Pat. No. 3,936,549. A doctor blade 11, held by a blade holder 22, is typically made of a metal softer than that of the surface of the gravure cylinder 10, and wipes any excess liquid from the surface of the gravure cylinder 10 such that only the engraved areas hold liquid. The gravure cylinder 10 then delivers a precise amount of liquid that exits via a drainage port 26 to a web 18 or other receiving surface upon contact with the engraved areas. The transfer typically occurs in a nip 20 between the gravure cylinder 10 and an impression roller 19 with an elastomeric cover (not shown) that serves as a backing for the web 18. The impression roller 19 presses web 18 against gravure cylinder 10 to create a small area of contact. Alternatively, web 18 can be drawn against gravure cylinder 10 by its tension to create a nip 20.
Dye donor ribbon for thermal printers is manufactured by the gravure coating method. The dye donor ribbon has discrete patches of cyan, magenta, and yellow dyes that produce a color hardcopy when transferred to a receiving paper by a thermal printing head. To make the dye donor ribbon, a subbing layer is first applied to both sides of a plastic web to promote adhesion of subsequent layers. Solutions of dye and binder and registration ink are then applied to the subbed web in discrete patches by a series of gravure coating stations. At another coating station, a slip layer is applied to the back surface of the web to prevent the donor ribbon from sticking to the print head.
The solution of tetra-n-butyl titanate in n-propylacetate is used to make the subbing layer for thermal donor ribbon, as disclosed in U.S. Pat. No. 4,737,486, is an example of a highly reactive and volatile liquid of low viscosity. Tetraalkyl titanates undergo hydrolysis to form an inorganic polymer of high molecular weight and an alcohol biproduct, and so they scavenge surface water. Water vapor in the atmosphere is a source of surface water. According to the General Brochure for DuPont™ Tyzor® organic titanates, the rate of hydrolysis depends upon the size and complexity of the alkyl group, and the presence of alcohols can retard the reaction. Solvents or co-solvents that can be used to control reaction rate include n-butanol, sec-butanol and isopropanol.
Another consideration in choice of solvent is its volatility within the coating zone. Evaporation of the solvent in the coating zone increases the concentration of the solute so that more solute is applied to the web than desired. The solvent lost to evaporation must be replenished to maintain a substantially constant concentration of solute, therefore, representing a cost. On the other hand, a solvent of too low volatility can unacceptably reduce the drying rate of the coating.
If the coating composition undergoes substantial hydrolysis before coating, the functionality of the subbing layer is compromised. In that case, the coating operation must be stopped and the coating liquid replaced. Acceptably slow reaction times cannot always be obtained through choice of solute and solvent. Therefore, in the prior art, shown in FIG. 1, that utilizes a pan feed, the pan is extended to enshroud the gravure cylinder in such cases, and a dry, inert gas such as nitrogen is injected through gas distribution means 28 and 30 to replace the air in contact with the coating liquid as shown in FIG. 1. By reducing or eliminating atmospheric water vapor, the hydrolysis reaction takes place substantially in the coated layer during drying, and long, continuous production runs are possible. The gas distribution means for injecting the inert gas can be any of many such means known in the art, including a die, a conduit with small holes or narrow slits, or a conduit with a side that is perforated or a porous plate.
The use of nitrogen or other inert gases is disclosed in U.S. Pat. No. 4,600,608. When drying takes place in the atmosphere, the concentration of solvent vapor must be maintained below that where combustion can occur. When drying takes place in an inert gas such as nitrogen, however, the concentration of solvent vapor can safely reach saturation. The use of nitrogen or other inert gas that has been saturated with solvent is known in the art in the context of preventing undesired drying in a coating zone, as disclosed in U.S. Pat. No. 6,426,119. For example, drying on the lip of a coating die can produce a buildup of solute and a streaked coating. In prior art literature, nitrogen is also employed to overcome limitations on solvent vapor concentration imposed by explosive mixtures with atmospheric oxygen.
The pan feed application is relatively simple, but has limitations and disadvantages; the most prominent disadvantage is that the liquid entrains the ambient gas at high coating speeds with the result that the gravure cylinder 10 is not completely wetted and the cells are not completely filled such that imperfections and skips in the coating occur. Poorly wetted areas have detrimental effects on the doctor blade 17. The pan feed method supplies liquid to the gravure cylinder far in excess of that needed to fill the engraving. Large amounts of liquid are rejected by the doctor blade 17, resulting in spraying and splashing of the liquid, particularly from the ends of the gravure cylinder 10 causing possible contamination of the ribbon (not shown). Furthermore, the pan feed method is prone to flow lines and flow patterns that produce a non-uniformly coated ribbon.
Other methods of supplying the gravure cylinder with coating liquid are known in the art. These include a freely falling curtain as in U.S. Pat. Nos. 5,681,389 and 6,228,431, and a jet as in section 12d.2.3 and FIG. 12d.4(b) on page 642 of the book, “Liquid Film Coating” (S. F. Kistler and P. M. Schweizer, eds., Chapman & Hall, New York, 1997). A curtain or jet feed method for filling the cells of a gravure cylinder mitigates most of the limitations and disadvantages of the pan feed method. Higher coating speeds are obtainable before the onset of air entrainment. Spraying and splashing are reduced, because a lesser, controlled amount of liquid is applied to the gravure cylinder. Flow lines and patterns associated with a pool are eliminated. However, providing a blanket of inert gas for a curtain or jet is far more difficult than providing a blanket of inert gas for a pan feed apparatus, because of the sensitivity of a thin sheet of liquid to ambient disturbances. The greater flow of gas required promotes turbulence that can easily disrupt, deflect, or even rupture the curtain or jet. As established later in Example 1, a straightforward combination of a die 12 for jet feed with the prior art gravure apparatus of FIG. 1, as shown in FIG. 6, does not adequately control hydrolysis rate at gas supply rates that do not disrupt jet 11. Indeed, in prior art, including U.S. Pat. Nos. 3,508,947, 4,287,240, 5,114,759, and 5,976,630, considerable effort is expended to reduce air currents around a curtain and reduce the difference in pressures on the two faces of the curtain. Moreover, as already recited, drying at the edges of the coating liquid on the die or other curtain or jet formation means when the solvent is volatile is a problem that injection of an unsaturated inert gas exacerbates. So, prior art discourages the injection of unsaturated inert gas near a curtain or jet. The need, therefore, exists for a gravure coating method wherein a curtain or jet wets the gravure cylinder with a coating liquid that is reactive to the atmosphere and wherein the coating liquid is blanketed by an inert gas in a non-disruptive manner.